Referring now to the drawing, wherein like reference numerals refer to like elements throughout, FIG. 1 illustrates the electromagnetic spectrum on a logarithmic scale. The science of spectroscopy studies spectra. In contrast with sciences concerned with other parts of the spectrum, optics particularly involves visible and near-visible light—a very narrow part of the available spectrum which extends in wavelength from about 1 mm to about 1 nm. Near visible light includes colors redder than red (infrared) and colors more violet than violet (ultraviolet). The range extends just far enough to either side of visibility that the light can still be handled by most lenses and mirrors made of the usual materials. The wavelength dependence of optical properties of materials must often be considered.
Absorption-type spectroscopy offers high sensitivity, response times on the order of microseconds, immunity from poisoning, and limited interference from molecular species other than the species under study. Various molecular species can be detected or identified by absorption spectroscopy. Thus, absorption spectroscopy provides a general method of detecting important trace species. In the gas phase, the sensitivity and selectivity of this method is optimized because the species have their absorption strength concentrated in a set of sharp spectral lines. The narrow lines in the spectrum can be used to discriminate against most interfering species.
In many industrial processes, the concentration of trace species in flowing gas streams and liquids must be measured and analyzed with a high degree of speed and accuracy. Such measurement and analysis is required because the concentration of contaminants is often critical to the quality of the end product. Gases such as N2, O2, H2, Ar, and He are used to manufacture integrated circuits, for example, and the presence in those gases of impurities—even at parts per billion (ppb) levels—is damaging and reduces the yield of operational circuits. Therefore, the relatively high sensitivity with which water can be spectroscopically monitored is important to manufacturers of high-purity gases used in the semiconductor industry. Various impurities must be detected in other industrial applications. Further, the presence of impurities, either inherent or deliberately place, in liquids have become of particular concern of late.
Spectroscopy has obtained parts per million (ppm) level detection for gaseous contaminants in high-purity gases. Detection sensitivities at the ppb level are attainable in some cases. Accordingly, several spectroscopic methods have been applied to such applications as quantitative contamination monitoring in gases, including: absorption measurements in traditional long pathlength cells, photoacoustic spectroscopy, frequency modulation spectroscopy, and intracavity laser absorption spectroscopy. These methods have several features, discussed in U.S. Pat. No. 5,528,040 issued to Lehmann, which make them difficult to use and impractical for industrial applications. They have been largely confined, therefore, to laboratory investigations.
In contrast, cavity ring-down spectroscopy (CRDS) has become an important spectroscopic technique with applications to science, industrial process control, and atmospheric trace gas detection. CRDS has been demonstrated as a technique for the measurement of optical absorption that excels in the low-absorbance regime where conventional methods have inadequate sensitivity. CRDS utilizes the mean lifetime of photons in a high-finesse optical resonator as the absorption-sensitive observable.
Typically, the resonator is formed from a pair of nominally equivalent, narrow band, ultra-high reflectivity dielectric mirrors, configured appropriately to form a stable optical resonator. A laser pulse is injected into the resonator through a mirror to experience a mean lifetime which depends upon the photon round-trip transit time, the length of the resonator, the absorption cross section and number density of the species, and a factor accounting for intrinsic resonator losses (which arise largely from the frequency-dependent mirror reflectivities when diffraction losses are negligible). The determination of optical absorption is transformed, therefore, from the conventional power-ratio measurement to a measurement of decay time. The ultimate sensitivity of CRDS is determined by the magnitude of the intrinsic resonator losses, which can be minimized with techniques such as superpolishing that permit the fabrication of ultra-low-loss optics.
At present, CRDS is limited to spectroscopic regions where high reflectivity dielectric mirrors can be used. This has significantly limited the usefulness of the method in much of the ultraviolet and infrared regions, because mirrors with sufficiently high reflectivity are not presently available. Even in regions where suitable dielectric mirrors are available, each set of mirrors only allows for operation over a small range of wavelengths, typically a fractional range of a few percent. Further, construction of many dielectric mirrors requires use of materials that may degrade over time, especially when exposed to chemically corrosive environments. Because these present limitations restrict or prevent the use of CRDS in many potential applications, there is a clearly recognized need to improve upon the current state of the art with respect to resonator construction.
The article by A. Pipino et al., “Evanescent wave cavity ring-down spectroscopy with a total-internal reflection minicavity,” Rev. Sci. Instrum. 68 (8) (August 1997), presents one approach to an improved resonator construction. The approach uses a monolithic, total internal reflection (TIR) ring resonator of regular polygonal geometry (e.g., square and octagonal) with at least one convex facet to induce stability. A light pulse is totally reflected by a first prism located outside and in the vicinity of the resonator, creating an evanescent wave which enters the resonator and excites the stable modes of the resonator through photon tunneling. When light impinges on a surface of lower index of refraction that the propagation medium at greater than a critical angle, it reflects completely. J. D. Jackson, “Classical Electrodynamics,” Chapter 7, John Wiley & Sons, Inc.: New York, N.Y. (1962). A field exists, however, beyond the point of reflection that is non-propagating and decays exponentially with distance form the interface. This evanescent field carries no power in a pure dielectric medium, but attenuation of the reflected wave allows observation of the presence of an absorbing species in the region of the evanescent field. F. M. Mirabella (ed.), “Internal Reflection Spectroscopy,” Chapter 2, Marcel Dekker, Inc.: New York, N.Y. (1993).
The absorption spectrum of matter located at the totally reflecting surfaces of the resonator is obtained from the mean lifetime of a photon in the monolithic resonator, which is extracted from the time dependence of the signal received at a detector by out coupling with a second prism (also a totally reflecting prism located outside, but in the vicinity of, the resonator). Thus, optical radiation enters and exits the resonator by photon tunneling, which permits precise control of input and output coupling. A miniature-resonator realization of CRDS results and the TIR-ring resonator extends the CRDS concept to condensed matter spectroscopy. The broadband nature of TIR circumvents the narrow bandwidth restriction imposed by dielectric mirrors in conventional gas-phase CRDS. The work of A. Pipino et al. is only applicable to TIR spectroscopy, which is intrinsically limited to short overall absorption pathlengths, and thus powerful absorption strengths. In contrast, the present invention provides long absorption pathlengths and thus allows for detection of weak absorption strengths.
Various novel approaches to mirror based CRDS systems are provided in U.S. Pat. Nos. 5,973,864, 6,097,555, 6,172,823 B1, and 6,172,824 B1 issued to Lehmann et al., and incorporated herein by reference. These approaches teach the use of a near-confocal resonator formed by two reflecting elements or prismatic elements.
FIG. 2 illustrates a prior art CRDS apparatus 10. As shown in FIG. 2, light is generated from a narrow band, tunable, continuous wave diode laser 20. Laser 20 is temperature tuned by a temperature controller 30 to put its wavelength on the desired spectral line of the analyte. An isolator 40 is positioned in front of and in line with the radiation emitted from laser 20. Isolator 40 provides a one-way transmission path, allowing radiation to travel away from laser 20 but preventing radiation from traveling in the opposite direction. Single mode fiber coupler (F.C.) 50 couples the light emitted from laser 20 into the optical fiber 48. Fiber coupler 50 is positioned in front of and in line with isolator 40. Fiber coupler 50 receives and holds optical fiber 48 and directs the radiation emitted from laser 20 toward and through a first lens 46. First lens 46 collects and focuses the radiation. Because the beam pattern emitted by laser 20 does not perfectly match the pattern of light propagating in optical fiber 48, there is an inevitable mismatch loss.
The laser radiation is approximately mode-matched into a ring down cavity (RDC) cell 60. A reflective mirror 52 directs the radiation toward a beam splitter 54. Beam splitter 54 directs about 90%, of the radiation through a second lens 56. Second lens 56 collects and focuses the radiation into cell 60. The remaining radiation passes through beam splitter 54 and is directed by a reflective mirror 58 into an analyte reference cell 90.
The radiation which is transmitted through analyte reference cell 90 is directed toward and through a fourth lens 92. Fourth lens 92 is aligned between analyte reference cell 90 and a second photodetector 94 (PD 2). Photodetector 94 provides input to computer and control electronics 100.
Cell 60 is made from two, highly reflective mirrors 62, 64, which are aligned as a near confocal etalon along an axis, a. Mirrors 62, 64 constitute the input and output windows of cell 60. The sample gas under study flows through a narrow tube 66 that is coaxial with the optical axis, a, of cell 60. Mirrors 62, 64 are placed on adjustable flanges or mounts that are sealed with vacuum tight bellows to allow adjustment of the optical alignment of cell 60.
Mirrors 62, 64 have a high-reflectivity dielectric coating and are oriented with the coating facing inside the cavity formed by cell 60. A small fraction of laser light enters cell 60 through front mirror 62 and “rings” back and forth inside the cavity of cell 60. Light transmitted through rear mirror 64 (the reflector) of cell 60 is directed toward and through a third lens 68 and, in turn, imaged onto a first photodetector 70 (PD 1). Each of photodetectors 70, 94 converts an incoming optical beam into an electrical current and, therefore, provides an input signal to computer and control electronics 100. The input signal represents the decay rate of the cavity ring down.
FIG. 3 illustrates optical path within a prior art CRDS resonator 100. As shown in FIG. 3, resonator 100 for CRDS is based upon using two Brewster's angle retroreflector prisms 50, 52. The polarizing or Brewster's angle, ΘB, is shown relative to prism 50. Incident light 12 and exiting light 14 are illustrated as input to and output from prism 52, respectively. The resonant optical beam undergoes two total internal reflections without loss in each prism 50, 52 at about 45°, an angle which is greater than the critical angle for fused quartz and most other common optical prism materials. Light travels between prisms 50, 52 along optical axis 54.
Although, when compared with the other spectroscopy methods, ring down cavity spectroscopy is a simpler and less expensive to implement, it is still costly in that a ring down cavity spectroscopy system can cost on the order of many thousands of dollars per unit. In addition, conventional CRDS devices are prone to misalignment between the optical elements while being fabricated as well as during use.
To overcome the shortcomings of the known approaches to improved resonator construction, a new optic-fiber based optical resonator for CRDS is provided. An object of the present invention is to replace conventional fiber optic sensors with sensors having enhanced evanescent field portion, thereby providing a more sensitive fiber optic sensor.